Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12

Zero- and longitudinal-field muon-spin-rotation (mu SR) experiments were performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below T-c a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in PrPt4Ge12 below T-c=7.9 K. This remarkably high T-c for an anomalous superconductor and the weak and gradual change in T-c and of the related specific-heat anomaly upon La substitution in La1-xPrxPt4Ge12 suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs.